1. Field of the Invention
This invention relates to film scanners, and more particularly to frame-rate converting scanners for scanning a motion picture frame projected at a desired frame rate to product a television signal at a field rate that is not necessarily a whole number multiple of the projection frame rate.
2. Discussion Related to the Problem
Apparatus for scanning motion picture film to produce a television signal is well known. In such film scanning apparatus, a film frame is scanned in a direction generally perpendicular to the length of the film at a standard television line rate. The line scan is displaced in a direction generally parallel with the length of the film at a standard television field rate. In the past, the line scan has been accomplished by flying spot scanners employing Nipkow discs, movable reflecting elements, or cathode ray tubes. The use of a solid-state line sensing array as an alternative to a flying spot scanner to provide the line scanning function in a film scanner has been proposed. See D. T. Wright, "Solid-State Sensors; The Use of a Single Dimension 512-Element Array for Film Scanning", BBC Research Department Report No. 1973/32. The potential advantages of a line sensing array over a flying spot scanner are numerous, including: reduced size, weight, maintenance and cost.
These advantages make the solid-state line sensing array particularly attractive for use in a film scanner for displaying amateur movie film on a home television set. However, since standard motion picture frame rates are not the same as standard television field rates, the scanning apparatus must provide some form of frame-rate conversion, whereby the film is moved at a desired projection frame rate, but the scan of a frame is accomplished at the standard TV field rate.
In experiments performed by the BBC on a film scanner employing a line sensing array, frame-rate conversion was accomplished by projecting a film that was exposed at 24 frames per second, onto a solid-state line sensing array, at the slightly faster rate of 25 frames per second. The signal, generated by scanning a frame with the line sensing array, was held in a field store device and read out of the device two times in succession at twice the 25 frame per second rate, resulting in a TV signal having a 50 field per second rate. This is the standard field rate for European television Alternatively, it has been suggested to interpose a device such as a movable reflector or rotating multifacet mirror such as a reflecting polygon in the optical path between the film and the line sensing array, and to move the device so as to cause the line sensing array to scan each frame twice. See I. Childs and J. Sanders, "An Experimental Telecine Using a Line-Array CCD Sensor", SMPTE Journal, Vol. 87, No. 4, April 1978.
Projecting a 24 frame per second film at 25 frames per second introduces a projection rate error of about 4%. Unfortunately, amateur motion picture film, i.e. regular 8 and Super 8, is commonly exposed at 18 frames per second and the standard U.S. television field rate is 60 fields per second. If the 18 frame per second film were projected at 20 frames per second, and a 3 to 1 frame-rate conversion were effected in some way, a projection rate error of 11% would be introduced. While a 4% error may be acceptable, an 11% projection rate error would certainly be noticeable and objectionable. The present invention solves the problem of exact frame-rate conversion in a film scanner and includes the added advantage that various projection rates may be effected the scanner.